The present invention relates to a read-only memory (hereinafter called an ROM) and a method for manufacturing the same, and more particularly to a highly reliable ROM which can be quickly put into production, as in response to an order code from a customer, for example, and to a method for producing such ROM's.
As component elements of a ROM matrix, resistors, capacitors, diodes and the like are available. In the early digital computer age, an ROM employing simple diodes has been used. However, at present, it is a common practice to employ MOS type field-effect transistors. As micro computers have become more popular, an ROM forming one of its component elements has come to use a great number of codes.
Except for a programmable ROM (PROM) which can be electrically written and can be erased either electrically or by ultra-violet irradiation, general ROM's are called a mask ROM. In these mask ROM's, a selection of codes is achieved during manufacture either by selecting existing or non-existing MOS gates as memory cells transistors forming the ROM matrix or by selecting existing or non-existing contacts of the ROM. During manufacture, the selection is made by means of either a mask for the gates or a mask for the contacts.
However, in order to shorten the production period beginning with the reception of an order for ROM codes and ending with the delivery of products, it is a common practice to actually select the code at the manufacturing step of a process, which step is nearest to the completion in products. At present, the selection is generally achieved by making a combination of contacts.
Such a ROM comprises, for example, source regions extending in one direction along one major surface of a semiconductor substrate of one conductivity type. The source region has an opposite conductivity type. A plurality of drain regions are disposed, at predetermied intervals on both sides of the respective source regions. Gate electrodes consist of polycrystalline silicon and extend in the one direction over the one major surface and between the source and drain regions, via a gate insulator film. The gate electrode serves as a word line. A plurality of digit lines consist of, for instance, aluminum and extend in a direction at right angles to the one direction, over the gate electrodes and one major surface, via an insulating film, and among the drain regions arranged in a matrix pattern and digit wires. The digit lines are selected according to a code designation and are electrically connected through respective apertures in the insulating film.
In the above-described prior art ROM structure the code selection is made by means of a contact mask. The portion of the insulating film contacting an aluminum wiring is normally made of phosphorous glass. However, such phosphorous glass is subjected to the following reaction when there is a minute amount of water: EQU P.sub.2 O.sub.5 +3H.sub.2 O.fwdarw.2H.sub.3 PO.sub.4
resulting in a production of phosphoric acid, which causes a deterioration of the aluminum wiring, and so, this structure has very poor humidity resistance.
As one solution for this problem, a vapor-grown silicon oxide film (of 0.1-1.0.mu. thickness) is formed between the phosphorous glass and the aluminum wiring. This silicon oxide film is made denser through a heat treatment. The phosphorous glass layer is prevented from directly contacting the aluminum wiring. However, the insulating layer consisting of the phosphorous glass film and the silicon oxide film becomes thick at the contact portion. Therefore, new disadvantages arise such that the contact aperture etching becomes difficult and the aluminum wiring is opt to break at the contact portion.
In order to overcome the new disadvantages, the apertures for making the contacts have been formed by means of the same contact mask before and after the formation of the vapor-grown silicon oxide film. According to this method, upon the first formation of the apertures for contacts in the phosphorous glass film or the thermal oxidation film, the contact apertures are formed in a tapered form. Hence, after the vapor-grown silicon oxide film has been laid down, the step of the oxide film at the contact aperatures becomes small, that is, steep steps of the oxide film are reduced in number. An advantage is obtained in that there are fewer breaks of the aluminum wiring at the contact portions. On the other hand, there is a disadvantage since the process is very time-consuming because it includes the steps of forming contact apertures, forming a vapor-grown silicon oxide film and forming contact apertures. Thus, there is a long lead time between the delivery of the product and the reception of an order of ROM codes.